1. Field of the Invention
The present invention relates to a structure of a connecting portion of a fusion spliced optical fiber, and to a light monitor apparatus for monitoring the power of optical signals propagated along an optical fiber, and in particular relates to an optical fiber connecting portion structure, and a light monitor apparatus directed to light radiated from a fusion splice portion.
2. Description of the Related Art
Optical fibers have been developing as a medium for transmitting optical signals. As this medium there is for example a silica glass fiber and a plastic fiber which are used in optical communication systems. In optical communication systems, these are used in all kinds of optical parts constituting systems such as transmission lines. More specifically, there is for example; a laser diode (LD) which constitutes a signal light source, a photodiode (PD) which constitutes an optical receiver, an optical coupler which branches one part of the light, an optical switch which switches optical paths, a wavelength combining and branching filter used in wavelength multiplexing and demultiplexing of the optical signal, an optical isolator which transmits light in one direction, an optical filter which filters the light, and an optical amplification fiber which constitutes an optical amplifying medium. In order to assemble together these plurality of optical parts and complete an optical module of an optical modulator, or an optical amplifier, or the like, connection of the optical fibers is necessary.
General connection methods for optical fibers include for example, a method of physically connecting, and a fusion splice method which connects by heating a glass base material to a high temperature and melting. In the physical connection method, an optical fiber is reinforced with a ferrule of for example zirconia, glass, or metal, and connected using an adapter. In the fusion splice method, an electrical discharge is produced by an electrode, and the fibers are connected during the discharge, to thereby perform a fusion splice. A fusion splice apparatus which uses such a method is being put to practical used.
In general, in a silica glass fiber, in order to prevent breaking due to the glass surface being damaged, a UV coat formed using an ultraviolet ray (UV) curing resin is applied. Therefore, at the time of the fusion splice operation, this UV coat is temporarily removed, and the fusion splice performed, and after splicing, the connecting portion is again protected using a heat shrinkable tube or the like. Presently, a re-coat technique is adopted which again covers the temporarily removed UV coat to the same thickness using the UV curing resin (for example refer to Japanese Unexamined Patent Publication No. 2001-343548 and Japanese Unexamined Patent Publication No. H10-73729). This re-coat technique is an effective means in high density installing of optical fibers and connecting portions. The UV curing resin used in the abovementioned re-coat technique, is also used in a wide range of fields other than as a coating material for optical fiber, such as for example a lens forming material or optical adhesive, an adhesive for sticking of optical disks, a hard coat for LCD plastic films, a resin for three dimensional solid shapes, and so on. The UV curing resin as is apparent from the use application, has superior transmissivity with respect to light of visible light (480 to 800 nm) being wavelengths above that of UV rays (200 to 400 nm), and light of wavelengths (800 to 1650 nm) used in optical communication,
Incidentally, at the connecting portion of an optical fiber which is fusion connected by the above described conventional method, joint loss occurs due to eccentricity of the core, or mismatch of the mode field diameter. For example, joint loss in homogenous optical fibers is around 0.1 dB. Therefore, for example as shown in FIG. 7, at a fusion splice portion S of the optical fiber, due to the abovementioned joint loss, a part of an optical signal L1 propagated along a core 101 is radiated from the core 101 to a cladding 102. The light L1′ radiated to the cladding 102 is propagated along the optical fiber as a cladding mode. Then if another optical fiber is adjacent to the optical fiber in which the cladding mode is being propagated, a phenomena occurs where the light L1′ of this cladding mode leaks in to the other optical fiber.
FIG. 8 is an example of a measurement system which evaluates the proportion of coupling of light of the cladding mode with another optical fiber (cross talk), in the case where a fusion splice portion of an optical fiber is adjacent to another optical fiber. In this measurement system, as a light source 200, two types of light source having for example a wavelength of 980 nm and a wavelength of 1480 nm are used. Here, an intensity P1 [dBm] of the light which is propagated along an optical fiber F1 with a re-coat portion 104 which is transparent with respect to the light output from the light source 200, formed in the vicinity of a fusion splice portion S, is measured by an optical power meter 201, and an intensity P2 [dBm] of the light of a cladding mode coupled with another optical fiber F2 adjacent to the fusion splice portion S of the optical fiber F1, is measured with an optical power meter 202, and based on each of the measurement results, a cross talk amount [dB]=P1 [dBm]−P2 [dBm] is obtained. For the optical fiber F2 which couples the light of the cladding mode, a distance from the fusion splice portion S to the optical power meter 202 is L [cm].
FIG. 9 shows the measurement results related to the distance L [cm] and the cross talk amount [dB] in the measurement system of FIG. 8. In the range of L=5 to 30 cm, it is seen that with light of a wavelength of 980 nm, 50 to 67 dB of cross talk is produced, and with light of a wavelength of 1480 nm, 50 to 53 dB of cross talk is produced. Regarding these cross talk amounts, for example assuming a ratio of pumping light power to signal input power in the optical amplifier, then this corresponds to a level which leads to deterioration in the optical SN ratio.
As one method for preventing the occurrence of such cross talk at the fusion splice portion of the optical fiber, for example it is considered to use an optical fiber in which for example as shown at the top of FIG. 10, the UV coating has been subjected to coloring. However, also for an optical fiber in which the UV coating has been subjected to coloring, in the case where as shown at the bottom of FIG. 10, the fusion splice portion S′ is re-coated, if the conventional high transmissivity UV curing resin is used as the material of the aforementioned re-coat portion 104′, there is the possibility that the light radiated from the fusion splice portion S of the adjacent optical fiber will leak in.
FIG. 11 is a diagram showing a configuration example of a common optical amplifier. In this configuration example, pumping light output from a pumping light source (LD) 301, is supplied to an erbium doped fiber (EDF) 300 via a WDM coupler 302. Furthermore, a part of input light from an input terminal IN which is applied to the EDF 300 via an optical isolator 303 and the WDM coupler 302, is branched by a branching coupler 305, and monitored by an optical receiver 306, and a part of the output light from the EDF 300 which is transmitted to an output terminal OUT via an optical fiber 304 is branched by a branching coupler 307, and monitored by an optical receiver 308.
The above respective constituent components of the common optical amplifier, are each connected by means of an optical fiber having a fusion splice portion S. The constituent components and the connection optical fiber are modularized in an installation state as shown for example in the schematic diagram of FIG. 12. In such an installation state, for example in the case where the fusion splice portion S of the optical fiber between the pumping light source 301 and the WDM coupler 302, and the fusion splice portion S of the optical fiber between the branching coupler 305 and the optical receiver 306 are adjacent, a part of the pumping light leaks in to the input module side via the fusion splice portion S, so that there is the possibility that the optical SN ratio of the input signal light monitored by the optical receiver 306 is deteriorated. More specifically, for example if the input optical power to the optical amplifier is −30 dBm, and the loss of the branching coupler 305 is 13 dB, the power of the input monitor light which reaches to the input light monitoring optical receiver 306 becomes −43 dBm. At this time, if the power of the pumping light is 20 dBm, and the cross talk amount between the respective optical fibers on the pumping light side and the input monitor light side is 50 to 60 dB, pumping light of −40 to −30 dBm leaks in to the optical receiver 306. Consequently, a leak in component of pumping light with a larger power than that of the input monitor light is input to the optical receiver 306. As a result deterioration occurs in the monitor accuracy of the input light to the optical receiver.
An imperfect alignment splice technique which intentionally produces a loss in the fusion splice portion of the optical fiber, is also being put to practical use. If this imperfect alignment splice technique is used, then a loss of approximately 3 dB can be easily produced. In the case where this imperfect alignment splice technique is applied, then even more light is propagated along the cladding mode, and there is the possibility of this leaking in to other optical fibers, and an even greater deterioration in the optical SN ratio occurring.
On the other hand, regarding the light radiated from the aforementioned fusion splice portion S of the optical fiber, considering this from another view point, it is also possible to use this as monitor light or the like for monitoring the power of the signal light propagated along the optical fiber. However, so far there has not yet been a proposal related to a specific configuration for such a light monitor which actively uses the light radiated from the fusion splice portion S.